<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>css_选择器</title>
    <style>
        .xian{border:1px solid red;width:200px;height:200px;}
        #div1{background-color:blue;width:200px;height:200px;}
        .xian,#div1{border-radius:10px}
    </style>
</head>
<body>
<div class="xian"></div>
<div id="div1"></div>


</body>
</html>